1. Field of the Invention
The present invention relates to solar cells and particularly to Cu.sub.2 S-CdS thin film solar cells.
2. Description of the Prior Art
Present high output CdS thin film solar cells are made by vacuum depositing a CdS film on a conductive substrate, then converting its surface to Cu.sub.2 S by dipping it in a solution containing cuprous ions. Cuprous ions in the solution chemically interchange with and displace cadmium ions in the CdS crystal lattice thereby forming a very thin layer of Cu.sub.2 S with the same structure and lattice spacing as the CdS. However, the Cu.sub.2 S also forms in the boundaries between the CdS grains, penetrating about 10 to 15 microns or more into the CdS film, which is generally 25 to 30 microns thick. If the CdS layer is made thinner and approaches 15 microns or less in thickness, short circuits occur due to the Cu.sub.2 S penetrating therethrough down to the conductive substrate. If the ion exchange reaction is reduced to give much less penetration of the grain boundaries, then the Cu.sub.2 S layer formed is too thin for effective absorption of the incident light. While a thick CdS layer is not desired, it must be employed in such prior practice. The present invention solves this problem, thereby providing solar cells with much thinner CdS layers.